Method of detecting methane in the bore of a blowout preventer stack

ABSTRACT

The method of sensing methane type gases in the bore of a subsea blowout preventer stack comprising collecting a sample of the fluids in the blowout preventer stack into a vacuum chamber, expanding the volume of the chamber to allow the methane type gases in the sample to evaporate either expand or evaporate, and then measuring the change in pressure before and after the expansion to determine the characteristics of the sample.

TECHNICAL FIELD

This invention relates to the method of detecting methane or other gasesin the bore of a subsea stack before they enter the lower pressuredrilling riser and begin to dangerously expand, tending to blow thedrilling mud out of the riser.

BACKGROUND OF THE INVENTION

When drilling an oil or gas well into the seafloor, drilling mud iscirculated down a central drill pipe, goes through a drill bit at thebottom where the hole is being drilled, and then circulates back upthrough the annular area between the central drill pipe and the hole.The hole consists of a lower portion which is the size of the drill bitdoing the drilling, up the slightly larger bore of the last casingstring landed, through the considerably larger bore of the subseablowout preventer stack at the seafloor, and finally up the slightlylarger drilling riser all the way to the surface. The outer diameter ofthe central drill pipe is relatively consistent above a short group ofoversize drill collars at the bottom. The velocity of the drilling mudslows each time the hole it is in becomes larger, and the pressure isreduced as the depth is decreased. What this generally means is thatfree gas can travel up quickly to the area of the seafloor blowoutpreventer slack, and then slowly moves up to the surface.

When a substantial volume of methane type gases enters the wellboreduring the drilling process and arrive up to the level of a seafloorblowout preventer stack, they can be so small as to not be noticeable inmost cases.

If you take the case of a blowout preventer stack in 10,000 feet depthsand presume a volume of gas the size of a ping pong ball (1.57″diameter) is in the bore. When the gas gets to the ocean surface it'svolume will be 20% bigger than two basketballs (9.4″ diameter each),given a typical drilling mud weight.

As the drilling mud travels up the bore of the drilling riser above theseafloor blowout preventer stack, additional gases may come out ofsolution and both the gas already out of solution and that which comesout of solution expand rapidly. What coming out of solution is can beeasily seen by a soft drink bottle or a bottle of champagne. An unopenedbottle will show no gas bubbles, and is under some pressure inside. Whenopened and the pressure is thereby reduced, suddenly some gas bubblesappear.

As if gas expanding from the size of one ping pong bails to more thantwice the size of a basketball isn't bad enough, gas coming out ofsolution basically expands from nothing to potentially a very largesize.

This has been a problem as long as oil and gas wells have been drilled.The best detection method presently is watching the level of thedrilling mud pits at the surface. During drilling you would expect themud to circulate down and back up at generally the same volume,considering that you are drilling a deeper hole all the time which takesadditional drilling mud to fill. But you know how fast you are drillingand can compensate for that. If your mud pit level is increasing, itwill tell you that something is entering the well bore, with a goodchance that it is gas. If your mud pit is falling more than what isexpected from your deepening the hole, you are “losing circulation”,generally meaning drilling mud is seeping into the formations. As almostevery blowout will tell you, including the 2009 Macondo blowout in theGulf of Mexico, there was not an adequate system for detecting problems.Additionally, when the mud pits have increased enough to detect it, ahigh volume of gas has already entered the drilling riser. Means forearlier detection have long been needed.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide a method of detecting freegas within the drilling mud at subsea locations such as a seafloorblowout preventer stack before it has a chance to expand and can bedirected out the higher pressure choke or kill lines.

A second object of this invention is to provide a method of defectingwhether gas in solution the drilling mud at subsea locations such as aseafloor blowout preventer stack will come out of solution at the lowerpressures seen as the drilling mud moves up towards the surface of theocean.

A third object of this invention is to regularly monitor this situationin a why which can be reported in real time to the personnel at thesurface and/or on land.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a system of subsea equipment utilizing a sensor ofthis method.

FIG. 2 is a half section of a portion of the blowout preventer stack ofFIG. 1 showing the basic components of the sensor.

FIG. 3 is a graph showing the anticipated pressures within the vacuumchamber of the sensor generally under the conditions of no methane inthe bore and with methane in the bore.

FIG. 4 is a half section similar to FIG. 2 showing the sensor with avacuum drawn on tire vacuum chamber and being monitored as illustratedon FIG. 2.

FIG. 5 is a half section similar to FIG. 4 showing the sensor with thevacuum having been collapsed.

FIG. 6 is a half section similar to FIG. 5 showing the sensor showingthe tested charge of fluid being at least partially removed.

FIG. 7 is a half section similar to FIG. 6 showing the sensor showingthe new charge of fluid starting to be drawn.

FIG. 8 is a half section similar to FIG. 7 showing the sensor showingthe new charge of fluid having been fully drawn.

FIG. 9 is a half section similar to FIG. 8 showing the sensor with thenew vacuum having been drawn and the vacuum being monitored, making itidentical to FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, a view of a complete system for drilling subseawells 20 is shown in order to illustrate the utility of the presentinvention. The drilling riser 22 is shown with a central pipe 24,outside high pressure fluid lines 26, and cables or hoses 28.

Below the drilling riser 22 is a flex joint 30, lower marine riserpackage 32, lower blowout preventer stack 34 and wellhead 36 landed onthe seafloor 38.

Below the wellhead 36 it can be seen that a hole was drilled for a firstcasing string 40, that first casing string 40 was landed and cemented inplace, a hole drilled through the first string for a second string, thesecond string 42 cemented in place, and a hole is being drilled for athird casing string by drill bit 44 on drill string 46.

The lower blowout preventer stack 34 generally comprises a lowerhydraulic connector 48 for connecting to the subsea wellhead system 36,usually 4 or 5 ram style blowout preventers, an annular preventer, andan upper mandrel for connection by the connector on the lower marineriser package 32, which are not individually shown but are well known inthe art.

Below outside high pressure fluid line 26 is a choke and kill (C&K)connector 50 and a pipe 52 which is generally illustrative of a choke orkill line. Pipe 52 goes down to valves 54 and 56 which provide flow toor from the central bore of the blowout preventer stack as may beappropriate from time to time. Typically, a kill line will enter thebore of the blowout preventers below the lowest ram and has the generalfunction of pumping heavy fluid to the well to overburden the pressurein the bore or to “kill” the pressure. The general implication of thisis that the heavier mud cannot be circulated into the well bore, butrather must be forced into the formations. A choke line will typicallyenter the well bore above the lowest ram and is generally intended toallow circulation in order to circulate heavier mud down the drill pipeinto the well to region pressure control of the well. Normal circulationis down the drill string 46, through the drill bit 44, up the annulararea between the drill pipe and the casing, and up the choke line.

In normal drilling circulation the mud pumps 60 take drilling mud 62from tank 64. The drilling mud will be pumped up a standpipe 66 and downthe upper end 68 of the drill string 46. It will be pumped down thedrill string 46, out the drill bit 44, and return up the annular area 70between the outside of the drill string 46 and the bore of the holebeing drilled, up the bore of the casing 42, through the subsea wellheadsystem 36, the lower blowout preventer stack 34, the lower marine riserpackage 32, up the drilling riser 22, out a bell nipple 72 and back intothe mud tank 64.

During situations in which an abnormally high pressure from theformation has entered the well bore, the thin walled central pipe 24 istypically not able to withstand the pressures involved. Rather thanmaking the wall thickness of the relatively large bore drilling riserthick enough to withstand the pressure, the flow is diverted to a higherpressure choke line or outside fluid line 26. It is more economical tohave a relatively thick wall in a small pipe to withstand the higherpressures than to have the proportionately thick wall in the largerriser pipe.

When higher pressures are to be contained, one of the annular or ramBlowout Preventers are closed around the drill pipe and the flow comingup the annular area around the drill pipe is diverted out through chokevalve 54 into the pipe 52. The flow passes up through C&K connector 50,up pipe 26 which is attached to the outer diameter of the central pipe24, through choking means illustrated at 74, and back into the mud tanks64.

Valve 54 which would be called a choke or kill valve (probably calledchoke in that position) is shown mounted on the side 90 of lower blowoutpreventer stack 34 and having a bore 92 communicating with the bore ofthe blowout preventer stack. On the opposite side of the tower blowoutpreventer stack another similar communicating bore 94 is provided andthe methane sensor assembly 96 is installed in this bore 94. Blowoutpreventer stack 34 has an internal bore 98.

Referring now to FIG. 2, blowout preventer stack 34 is shown with side90 and bore 98, communicating bore 94, and seal ring 100 sealinglyengaging flange 102 of methane sensor assembly 96. Filter 106 is held inplace by retainer ring 108. The general circulation path through methanesensor assembly 96 is illustrated by arrows 110-122 as will be furtherdescribed.

Filter 106 restricts particle size coming into the methane sensorassembly at arrow 110 and is continually cleaned by the reverse flow atarrow 120. Bore 124 leads to check valve 126, bore 128 leads to checkvalve 130 which checks the flow in the opposite direction as check valve126. Check valve 130 leads to vacuum chamber cavity 132 which leads tobore 134, leading to check valve 136, leading to bore 138, leading tobore 140, and back to filter 106. Drilled hole 142 leads to pressuretransmitter 144. Piston 146 is part of cylinder 148, with suitablefittings 150 for operation.

Referring now to FIG. 3, a graph is shown which generally shows theobjective of the methane sensor assembly. The numbers 4-9 at the bottomindicate the figure numbers which follow as the operations proceed inone full cycle from that shown in FIG. 4 to FIG. 9. Area 160 betweenindicates the normal pressure in the blowout preventer stack 34. Area162 indicates the anticipated pressure in the vacuum chamber cavity 132read by pressure transmitter 144 when there is no methane type gases inthe bore, and is likely the pressure signature of the water in thedrilling mud. Area 164 indicates the anticipated pressure in vacuumchamber cavity 132 when methane type gases are in the bore. Areas 166and 168 are transitional pressures while the vacuum is being pulled.Areas 170 and 172 are simply continuations of lines 162 and 164 into thenext cycle of operations. Areas 174 and 176 are transitional pressuresas the vacuum is being released. The distinction in the pressuresignatures will be utilized to determine that something different ishappening in the fluids within the blowout preventer stack 34.

The reason for this distinction in the pressures of the water in thewater based drilling muds and the invading methane gas is that waterboils and starts to put off pressures when it boils at 212 degrees F. or100 degrees C. Methane boils and starts to put off significant pressuresat −257 degrees F. or −162 degrees C. The vapor pressure of methane at70 degrees F. or 21 degrees C. is 4,641 p.s.i, or 32,000 kPa. This meanspure methane will not be a gas at all until the drilling mud is lessthan that pressure in typical drilling mud that would be (4641p.s.i.)/(0.465 lb/ft gradient times 1.33 heavier mud)=7487 feet. Thatmeans that pure methane will not evaporate before that depth, which is alittle deeper than the 6000 feet for the Macondo blowout. Otherpotential gases can come out at higher or lower pressures and depths.

Referring now to FIG. 4, methane sensor assembly is shown with a vacuumpuked and simply reading the pressure in vacuum chamber cavity 132 for aperiod of time, likely a couple of minutes. There is no flow occurringwithin the methane sensor assembly 96, however, flow 180 continueswithin the bore 98 of blowout preventer stack 34. Piston 146 is moved tothe right against a hard stop 182.

Referring now to FIG. 5, piston 146 has moved to the left to contact theend 190 of check valve 130 in making this movement, the vacuum iscollapsed but nothing else has happened. This is illustrated by areas174 and 172 of FIG. 3. The left side of FIG. 6 shows an end view of themethane sensor assembly 96 as it would be seen from inside the bore ofthe lower blowout preventer stack 34 having a central filter 106 and twosecuring bolts 192. Dashed lines show the hidden location of checkvalves 126, 130, and 136.

Referring now to FIG. 6, piston 146 has continued to move to the leftpushing check valve 130 open and expelling the tested fluids alongarrows 116 to 122 through filter 106, cleaning filter 106 as it flows inthis direction.

Referring now to FIG. 7, the movement of piston 146 is being reversed inthe opposite direction and beginning to draw fresh drilling fluids intothe methane sensor assembly along arrows 110 and 112 through filter 106.At this time check valve continues to be mechanically held open bypiston 146 and check valve 126 automatically opens by the flow.

Referring now to FIG. 8, piston 146 has moved to the right until thecheck valve 130 is closed and so the maximum amount of new drillingfluids have been pulled into the cavity.

Referring now to FIG. 9, piston 146 moves to the right against hard stop182 and a fresh vacuum has been drawn in vacuum chamber cavity 132, oris in other words in the same condition as FIG. 4. The pressuredifferential between the blowout preventer stack bore pressure is nowheld back by check valves 130 and 136.

By following this process, a fresh batch of drilling mud is drawn intothe methane sensor assembly, tested, documented, and expelled asfrequently as desired. The data is communicated to the surface formonitoring and analysis, with appropriate alarms sounding and sent whena change occurs.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

That which is claimed is:
 1. The method of sensing gas in the bore of asubsea blowout preventer stack, comprising collecting a sample of thefluids in said blowout preventer stack into a chamber, expanding thevolume of said chamber to allow said gas in said sample to evaporateeither expand or evaporate, and measuring the change in pressure beforeand after said expanding to determine the characteristics of saidsample.
 2. The method of claim 1 further comprising comparing saidcharacteristics with the characteristics of known fluids and gases todetermine what gases gasses are present in said sample.
 3. The method ofclaim 1 further comprising alternately drawing said sample of fluidsinto a test chamber and expelling said sample of fluids from said testchamber.
 4. The method of claim 3 further comprising providing a pistonto alternately draw said sample of fluids into a test chamber andexpelling said sample of fluids from said test chamber.
 5. The method ofclaim 3 further comprising drawing said sample of fluids through afilter and expelling said sample of fluids through the same filter toclean said filter.
 6. The method of claim 1 further comprising drawingsaid sample of fluids through a first check valve and expelling saidsample of fluids through a second check valve causing said sample offluids to be at least partially refreshed on each cycle.
 7. The methodof claim 6 further comprising opening a third check valve with themovement of a piston in a first direction.
 8. The method of claim 6further comprising closing said third check valve with the movement ofsaid piston in a second direction allowing a vacuum to be drawn byfurther movement is said second direction.
 9. The method of claim 1further comprising said gas is a methane type gas.
 10. The method ofclaim 1 further comprising said gas is methane.
 11. The method ofsensing methane type gases in the bore of a subsea blowout preventerstack, comprising collecting a sample of the fluids in said blowoutpreventer stack into a chamber; expanding the volume of said chamber toallow said methane type gases in said sample to evaporate either expandor evaporate, and measuring the change in pressure before and after saidexpanding to determine the characteristics of said sample.
 12. Themethod of claim 11 further comprising comparing said characteristicswith the characteristics of known fluids and gases to determine whatgases gasses are present in said sample.
 13. The method of claim 11further composing alternately drawing said sample of fluids into a testchamber and expelling said sample of fluids from said test chamber. 14.The method of claim 13 further comprising providing a piston toalternately draw said sample of fluids into a test chamber and expellingsaid sample of fluids from said test chamber.
 15. The method of claim 13further comprising drawing said sample of fluids through a filter andexpelling said sample of fluids through the same filter to clean saidfilter.
 16. The method of claim 11 further comprising drawing saidsample of fluids through a first check valve and expelling said sampleof fluids through a second check valve causing said sample of fluids tobe at least partially refreshed on each cycle.
 17. The method of claim16 further comprising opening a third check valve with the movement of apiston in a first direction.
 18. The method of claim 16 furthercomprising closing said third check valve with the movement of saidpiston in a second direction allowing a vacuum to be drawn by furthermovement is said second direction.
 19. The method of claim 11 furthercomprising said gas is a methane type gas.
 20. The method of claim 1further comprising said gas is methane.